1. Field of the Invention
The present invention relates to waveform playback devices and, more particularly, to active noise cancellation using waveform playback devices.
2. Description of the Relevant Art
Waveform playback devices, such as wavetable synthesizers, produce sound by using samples, or recordings, of instruments. Sound is produced by pitch shifting and looping samples to produce the desired sound. Pitch shifting and looping vary note pitch and duration of a finite number of samples to produce the desired sound. Other waveform playback devices, such as streaming wave playback devices, produce sound from a continuous data stream. The continuous data stream is a stream of digital data representing an audio signal.
Wavetable synthesizers use stored recordings of instruments to produce sound. The group of stored recordings is referred to as a patch set. A patch set typically comprises recordings from a plurality of instruments. Each patch comprises one or more recorded samples of an instrument. For example, a sample may be one note of a particular instrument. Wavetable synthesizers produce sound by pitch shifting and looping one or more samples. For example, a patch set may include a plurality sound recordings from a plurality of musical instruments such as a piano, a string instrument, and a trumpet. Each patch consists of one or more samples of an instrument, e.g., one or more notes of a piano. To synthesize the sound of a piano, a piano sample is selected. The selected sample is pitch shifted and looped to produce the desired sound. Multiple notes of the piano may be simultaneously synthesized. Multiple instruments may be synthesized by using samples from different patches. Sounds other than musical instruments may be synthesized by adding non-musical instrument patches to the patch set of the wavetable synthesizer.
A wavetable synthesizer may interface with a processor. The processor downloads the information necessary for the wavetable to synthesize the desired sound. The processor may specify the sample to use, frequency shifting data, looping data and amplitude data. The synthesization of the sound may be then performed by the wavetable independent of the processor. One standard for enabling a processor to provide commands to a wavetable synthesizer is referred to as MIDI (Musical Instrument Digital Interface).
Noise cancellation is a method of reducing the noise perceived by a listener. Noise cancellation is typically accomplished by outputting a noise cancellation signal that is the inversion of the noise that is desired to be cancelled. The noise and the noise cancellation signal combine to form a DC signal that is inaudible to the listener. One popular method for performing noise cancellation is to input a noise signal indicative of the noise that is desired to be cancelled, invert the noise signal and output the inverted noise signal. Although some delay is inherent in inputting, inverting and outputting the noise signal, the delay can be minimized such that a combination of the original noise signal and the inverted noise signal is substantially a DC signal. Unfortunately, noise cancellation typically requires specialized hardware to invert the noise signal at a rate which introduces a sufficiently minimized delay. Alternatively, noise cancellation can be accomplished using a general purpose processor, such as a personal computer. Noise cancellation using a general purpose computer typically requires a significant amount of processing power to sample and invert the noise. What is desired is a system and method for noise cancellation that minimizes the processing power necessary to calculate noise cancellation signals and does not require specialized hardware.